1. Field of the Invention
This invention relates to a process and apparatus for gas glow discharge, particularly hydrocarbon gas glow discharges at about atmospheric pressure with high conversion of methane to acetylene and efficient conversion of electrical energy into chemical conversions. This process may be used for high efficiency transfer of energy in the conduct of chemical reactions, such as a pretreatment for hydrocarbon gases, such as natural gas, to provide increased flame radiation and stability upon combustion. The hydrocarbon gas glow discharge according to this invention provides a high intensity white light and high volume light source. The glow discharge according to this invention may also be used to activate non-hydrocarbon gases, such as hydrogen, nitrogen, oxygen, ammonia, or silanes in molecular form to produce the corresponding atoms, ions, or excited species, such as free radicals.
2. Description of Related Art
Low pressure gas glow discharges and apparatus for their production are known from a number of U.S. Patents, for example U.S. Pat. Nos.: 2,787,730; 3,018,409; 3,035,205; 3,423,562; 4,830,492; 4,963,792; and 4,967,118. Glow discharge ignitors are known, for example, from U.S. Pat. Nos. 3,681,639 and 4,970,425.
Glow discharges are usually operable at subatmospheric pressure, typically less than about 20 Torr. When the pressure is increased, the glow discharge becomes an arc discharge. The two types of discharge are distinguished by their electrical characteristics and their mode of operation. A glow discharge operates at high voltage and low currents, while an arc discharge operates at low voltage and high currents. As the current is increased for a glow discharge, the discharge tends to cover more and more of the available cathode area until at some point the current density exceeds a critical value and the discharge suddenly becomes an arc. When this occurs, there is an abrupt drop in voltage and an increase in current. In the glow discharge, electrons are produced in the gas phase by ionization of neutral species by electrons accelerated by the electric field; in the arc discharge, the electrons are produced by copious emission of electrons from a hot cathode. Generally, the electrodes are not consumed in a glow discharge; while in an arc discharge, the cathode is consumed and must be replaced frequently.
Atmospheric pressure glow discharges in hydrogen have been described by Berman, C. H., Calcote, H. F. and Gill, R. J., Supersonic Combustion Enhancement by a Nonequilibrium Plasma Jet, Contract No. NAS1-18404, Final Report, AeroChem TP-467, (August 1987) and in air and hydrogen by Fan, H. Y., The Transition from Glow to Arc, Phys.Rev., 55, 769, (1939). In both of these cases, the electrode separation was very small, on the order of 1 mm. The observed discharges were identified as glow discharges because of the voltage/current relationship. However, in a normal glow discharge, the electrode spacing can be increased by increasing the applied voltage. In the above examples, this led to an arc discharge, so it may in fact be questioned as to whether a true glow discharge was observed.
Swirl has been used to stabilize arc discharges by moving the arc over the electrodes so they will not overheat or to spread the arc through gas flowing through the device. Jahn, Robert J., Physics of Electric Propulsion, pgs. 116-121, 140-141, McGraw-Hill Series in Missile and Space Technology, McGraw-Hill Book Company, New York, N.Y., (1968).
High power glow discharge lasers and apparatus for their production are known from a number of U.S. Patents, for example U.S. Pat. Nos.: 3,623,145; 3,704,428; 3,781,713; 3,982,209; 4,031,428; 4,335,462; and 4,604,752.
Production of acetylene from hydrocarbons by various methods is known: U.S. Pat. No. 2,719,184 teaching incomplete combustion with oxygen in a flame reaction; U.S. Pat. No. 2,799,640 teaching catalyzation by spark discharge; and U.S. Pat. No. 3,483,107 teaching utilization of radio frequency plasma jets. Laboratory scale electrical discharge conversion of methane to acetylene using a small spacing between electrodes and operated with very low flow rates has been described in Wiener, H. and Burton, M., Decomposition of Methane in an Electrical Discharge, J.Am. Chem. Soc., 75, 5815 (1953). Recycle of processed gases through an electrical discharge in the conversion of natural gas to acetylene has been described in Schoch, E. P., et al, Acetylene from Hydrocarbons, University of Texas Publication No. 5011, (June 1950). Pettyjohn, E. S., German Use of Natural Gas, A.G.A. Natl. Gas Dept. Proc., 33, (1946) describes an arc process